Orthogonal Frequency-Division Multiplexing (OFDM), also referred to as “multi-carrier modulation” (MCM) or “Discrete Multi-Tone Modulation” (DMTM), splits up and encodes high-speed incoming serial data, modulating it over a plurality of different carrier frequencies (called “subcarriers”) within a communication channel to transmit the data from one user to another. The serial information is broken up into a plurality of sub-signals that are transmitted simultaneously over the subcarriers in parallel.
By spacing the subcarriers frequencies at intervals of the frequency of the symbol to transmit, the peak power component of each modulated subcarriers line up exactly with zero power components of the other modulated subcarriers, thereby providing orthogonality (independence and separability) of the individual subcarriers. This allows a good spectral efficiency (close to optimal) and minimal inter-channel interference (ICI), i.e. interferences between the subcarriers.
For these reasons, OFDM is used in many applications. Many digital transmission systems have adopted OFDM as the modulation technique such as digital video broadcasting terrestrial TV (DVB-T), digital audio broadcasting (DAB), terrestrial integrated services digital broadcasting (ISDB-T), digital subscriber line (xDSL), WLAN systems, e.g. based on the IEEE 802.11 standard, cable TV systems, etc.
An OFDM signal is a signal with varying amplitude envelop, i.e. which carry information both in the amplitude and in the phase of the transmitted signal. In general, such a signal makes more complex the design of the transmitter according to the extent by which the amplitude varies. This extent is usually captured by the PAR parameter, defined as the peak-to-average power ratio. High PAR corresponds to modulation schemes with largely-varying amplitude envelop, whereas low PAR corresponds to modulation scheme where the amplitude envelop varies to a small extent.
High PAR modulation schemes are problematic to handle by transmission systems. For instance, in some systems, high peaks may be clipped by non-linear devices at the transmitter sides, causing undesirable effects such as high out-of-band activity (“regrowth”) and in-band distortion.
To prevent this phenomenon, the transmitter design should be carefully adapted, especially the analog-to-digital converter (ADC), but still some disadvantages are not negligible like a reduced efficiency of the radio frequency amplifier.
Despite these negative aspects, OFDM remains very interesting when weighted with its advantages, notably because of a much higher spectral efficiency.
According to the IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g standards, the theoretical maximum PAR is around 17 dB. In other words, the peak amplitude excursion of an 802.11a/g-compliant OFDM signal can be up to seven times larger than the average signal.
In order to prevent bad effects of a PAR at around 17 dB (i.e. to prevent distortion and to be able to reproduce the amplified output signal faithfully), the transmitter would need to avoid any undesired clipping, even during the peak excursions of the signal.
This requires the power amplifier to be designed so as to have minimal compression at the peak power. We can assume that a 1 dB compression is acceptable at peak power. However, most of the time, the power amplifier operates around the average amplitude (and not at the peak amplitude). This thus means that, most of the time, the amplifier operates at a power 17 dB lower than the 1 dB peak compression point, i.e. 7 times lower.
For instance, an inductively loaded class-A transmitter and power amplifier can achieve a maximum power efficiency of 50% (achieved when transmitting the maximum output swing). When it functions at 17 dB lower power below the 1 dB compression point, the best case achieved efficiency would be only 50/7=7%.
Of course, a transmitter amplifier with a power efficiency of 7% is not acceptable.
Some solutions try to improve the situation by optimizing the transmitter chain in order to achieve a better linearity and to obtain higher efficiency. The 1 dB compression point of the amplifier is increased and the amount of backoff required to achieve a particular error vector magnitude (EVM) is reduced. The backoff is defined as how much the signal level must be below the 1 dB compression point in order to reach a certain specified EVM.
These solutions are however not sufficient as they do not lead to efficient enough amplification. Furthermore, they do not simplify the design of the transmitter chain and, on the contrary, generally make it worse.